Worn Keypad Digits: How Visible Wear on a Code Lock Collapses Its Search Space

A keypad lock's code is only as secret as its buttons are unworn. When daily pressing polishes or fades a subset of digits, an observer learns which digits the code contains, and the search space collapses. This study counts the collapse exactly, by enumerating all 10,000 four-digit codes and retaining those built only from the worn digits. If wear reveals four distinct digits, only 256 candidate codes remain, a reduction by a factor of 39.1, exhaustible in 25.6 minutes at six seconds per attempt without a lockout. Three worn digits leave 81 candidates and two worn digits leave 16. A six-digit code with four worn digits leaves 4096 candidates, or about 6.8 hours of continuous trying, which restores practical safety even when fully worn. The rules that follow are to prefer six-digit codes, to choose codes whose digits repeat so wear reveals less, to clean or replace worn keypads, and to treat a lockout feature as worth more than extra digits against a patient attacker. SIMULATED, NOT MEASURED.